It is known to use an oval or flat sided tube which is formed to have round ends for mounting in openings in the headers at the upper and lower ends of a radiator. Such round end mounting arrangements are known to provide a strong tube-to-header joint. The larger the diameter of the ends, the more the area of contact between the two components and, hence, the stronger the joint. However, the diameter becomes limited when it is realized that predetermined minimum spaces must be maintained between adjacent openings in the headers, thus limiting the advantages of the round tube-to-header joints.